Redox variations and bioproductivity in the Ediacaran: Evidence from inorganic and organic geochemistry of the Corumbá Group, Brazil

Abstract

Stable isotope ratios combined with elemental compositions and molecular biomass data provide a powerful tool in Neoproterozoic palaeoenvironmental interpretations. Here, we report the results of an extensive organic and inorganic geochemical study performed in the Ediacaran sedimentary succession of the Corumbá Group (CG) from SW-Brazil, deposited in a shallow marine basin in southwestern Gondwana. This sedimentary succession and in particular the Tamengo Formation, a unit bearing metazoan fossils, has been investigated by means of stable isotopes from carbonates (δ13Ccar and δ18O) and associated organic matter (δ13Cker and δ15Nker) together with hydrocarbon distribution and concentrations of major, trace and rare earth elements (REE). A short post-glacial δ13Ccar negative excursion, interpreted as a period of water mixing, is recorded in the cap carbonates overlying diamictites of the Puga Formation, related to Gaskiers or end-Cryogenian glaciation. The overlying Tamengo Formation by contrast, represents a return to redox-stratified conditions in the basin before the Precambrian-Cambrian boundary. Two distinct biogeochemical modes alternate during deposition of Tamengo sediments: 1) an eutrophic, redox-stratified basin well defined by carbonaceous marls from the middle part of the unit but also recorded upwards, at the transition between bioclastic limestones and calcisiltites. 2) An anoxic basin well characterized in the shallow facies, particularly by bioclastic limestones of the upper Tamengo Formation. A positive Δ13Ccar–ker (Δ13Ccar–ker=δ13Ccar−δ13Cker) excursion of ~5‰ in the carbonaceous marls is explained by enhanced primary productivity in surface waters probably related to an increase of pCO2, nutrient supply and possibly also changes of the primary producer communities. Abundant pyrite, a biomarker distribution characterized by the occurrence of gammacerane and a low Pr/Ph ratio (~0.7) are also remarkable signatures of these facies, most probably associated with a sulfate-reducing microbial consortium in an anoxic and sulfidic (euxinic) environment. However, low concentrations in redox-sensitive trace elements in these facies suggest a largely oxygenated water column, thus constraining the euxinic setting to the sediments and/or bottom waters. The shallow-water bioclastic limestones record higher concentrations of redox-sensitive elements and ΣREE as well as a positive Ce anomaly supporting reducing conditions. Oxygenated conditions and dominance of eukaryotic algae characterize the overlying Guaicurus Formation. Principal component analysis (PCA) was used to assess the major geochemical associations. The most significant component combines parameters involved in primary production, such as P concentrations and δ13Cker values. The bio-chemostratigraphic variations in this part of SW-Gondwana point to a stratified ocean with oxic surface waters, alternating periods of high and low bioproductivity and anaerobic conditions at the bottom waters, in the aftermath of younger Neoproterozoic glaciations and close to the Precambrian–Cambrian boundary.